HKIE TRANSACTIONS, 2017 VOL. 24, NO. 1, 42–53 http://dx.doi.org/10.1080/1023697X.2016.1275825 Ductility design of RC columns. Part 2: extent of critical zone and confinement reinforcement Terry Y P Yuen a , J S Kuang b and Don Y B Ho c a Department of Civil Engineering, Bursa Orhangazi University, Bursa, Turkey; b Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, People’s Republic of China; c Structural Group, Ove Arup & Partners Hong Kong Ltd, Hong Kong, People’s Republic of China ABSTRACT The current Hong Kong concrete design code (HKConcrete2013), which is a non-seismic design code, includes provisions of the design of confinement zone in reinforced concrete (RC) columns. Whether or not these provisions can be adopted in the development of the new design code of structures for seismic resistance remains uncertain. A thorough scientific investigation with a comprehensive statistical analysis on the seismic behaviour of RC columns under different levels of the axial compression ratio (ACR) and concrete confinement has been conducted and pre- sented in the companion paper [1], where the use of the ACR limits for controlling the seismic performance of various types of RC columns is proposed. The present paper addresses the impor- tant aspect of the confinement zone length and the design of confinement reinforcement in RC columns under different levels of the ACR. The theoretical background of the confinement zone is presented and the requirements of the prevailing design standards for designing confinement reinforcement in RC columns are critically compared and discussed. Design recommendations for the development of new provisions on the design of the confinement zone in RC columns are made for (1) the revision of the extent of the critical regions and (2) the introduction of the required quantity of the confinement reinforcement. ARTICLE HISTORY Received 17 March 2016 Accepted 17 December 2016 KEYWORDS Reinforced concrete (RC) column; confinement zone; confinement reinforcement; axial compression ratio (ACR); ductility design 1. Introduction Hong Kong is classified as a region of moderate seis- micity and there is a rising concern over the seismic resistance capabilities of the non-seismically-designed buildings in the region. In 2004, the Hong Kong con- crete design code [2] introduced provisions on the design of the confinement zone in reinforced con- crete (RC) columns subjected to different levels of the axial compression ratio (ACR). The effects of the ACR on seismic behaviour were presented in a companion paper [1]. However, there is inconsistency in the spec- ified maximum design axial load that contradicts some existing provisions relating to RC column design. It can be seen that the confinement detailing provisions [2] are quite different from those stipulated by European design standards (EN 1998-1:2004 [3]), upon which most of the existing provisions in the Hong Kong code are largely based, as well as other prevailing design standards such as the Chinese seismic code (GB 50011- 2010 [4]) and the American Concrete Institute (ACI) concrete code (ACI 318-14 [5]). Furthermore, neither the requirements of the ACR limits nor the limiting material strains that are commonly specified in seis- mic design codes were adopted in the latest Hong Kong concrete design code (HKConcrete2013 [6]); therefore, it remains uncertain as to how well these provisions can be fitted into the Hong Kong’s moderate seismic situation. In order to develop new provisions on the ductil- ity design of RC columns in the coming design code of structures for seismic resistance in Hong Kong, there is a need to revisit the theoretical background and exper- imental results in this field of study. The effects of the ACR on the seismic performance of RC columns were addressed and design recommendations were made in a companion paper [1]. In the present paper, the design of the confinement zone in RC columns subjected to dif- ferent levels of the ACR – including the potential plastic hinge length and reinforcement detailing requirements – is thoroughly discussed on a scientific rationale. The requirements for confinement reinforcement stipulated in several prevailing design standards [3–9] – which concern the seismic performance on RC structures – are critically compared. Based on an analysis of the results, recommendations for the development of new provisions for the design of the confinement zone in RC columns are made. 2. Confinement and ductility of RC columns Catastrophic collapse of building structures is always a consequence of structural column failure, hence protecting columns from losing their load-carrying CONTACT Terry Y P Yuen [email protected] © 2017 The Hong Kong Institution of Engineers
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